Methods and Apparatuses for Managing Effluent Products in a Fuel Cell System

ABSTRACT

A water management system for a fuel cell having an anode chamber including a fuel, a cathode chamber in fluid communication with an oxidizing agent, and a proton conducting membrane electrolyte separating the chambers. The system includes a gas plenum, a first valve for controlling a first flow of a gas from the anode chamber into the gas plenum, and a second valve for controlling a second flow of the gas collected by the gas plenum into the cathode chamber. The first valve is opened allowing the first flow while the second valve is closed between the gas plenum and the cathode chamber so that effluent gas is collected in the gas plenum. When the amount of the effluent gas in the gas plenum reaches a predetermined value, the first valve is closed and the second valve is opened to allow the second flow.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to fuel cell systems, and moreparticularly, the invention relates to methods and apparatuses formanagement of effluent products produced during an electrochemicalreaction in a direct oxidation fuel cell system.

2. Background of the Invention

Fuel cells are devices in which an electrochemical reaction is used togenerate electricity. A variety of materials may be suitable for use asa fuel depending upon the materials chosen for the components of thecell and the intended application for which the fuel cell will provideelectric power.

Fuel cell systems may be divided into “reformer based” systems (whichmake up the majority of currently available fuel cells), in which fuelis processed to improve fuel cell system performance before it isintroduced into the fuel cell, and “direct oxidation” systems in whichthe fuel is fed directly into the fuel cell without internal processing.

Because of their ability to provide sustained electrical energy, fuelcells have increasingly been considered as a power source for smallerdevices including consumer electronics such as portable computers andmobile phones. Accordingly, designs for both reformer based and directoxidation fuel cells have been investigated for use in portableelectronic devices. Reformer based systems are not generally considereda viable power source for small devices due to size and technicalcomplexity of present fuel reformers.

Thus, significant research has focused on designing direct oxidationfuel cell systems for small applications, and in particular, directsystems using carbonaceous fuels including methanol, butanol, propanol,and formaldehyde. One example of a direct oxidation fuel cell system isa direct methanol fuel cell system. A direct methanol fuel cell powersystem is advantageous for providing power for smaller applicationssince methanol has a high energy density (providing compact energystorage), can be stored and handled with relative case, and because thereactions necessary to generate electricity occur under ambientconditions.

DMFC power systems are also particularly advantageous since they areenvironmentally friendly. The chemical reaction in a DMFC power systemyields only carbon dioxide and water as by products (in addition to theelectricity produced). Moreover, a constant supply of methanol andoxygen (preferably from ambient air) can continuously generateelectrical energy to maintain a continuous, specific power output. Thus,portable computers, mobile phones and other portable devices can bepowered for extended periods of time while substantially reducing andpotentially eliminating at least some of the environmental hazards andcosts associated with recycling and disposal of alkaline, Ni-MH andLi-Ion batteries.

The electrochemical reaction in a DMFC power system is a conversion ofmethanol and water to CO₂ and water. More specifically, in a DMFC,methanol in an aqueous solution is introduced to an anode chamber sideof a protonically-conductive, electronically non-conductive membrane inthe presence of a catalyst. When the fuel contacts the catalyst,hydrogen atoms from the fuel are separated from the other components ofthe fuel molecule. Upon closing of a circuit connecting a flow fieldplate of the anode chamber to a flow field plate of the cathode chamberthrough an external electrical load, the protons and electrons from thehydrogen atoms are separated, resulting in the protons passing throughthe membrane electrolyte and the electrons traveling through an externalload. The protons and electrons then combine in the cathode chamber withoxygen producing water. Within the anode chamber, the carbon componentof the fuel is converted by combination with water into CO₂, generatingadditional protons and electrons.

The specific electrochemical processes in a DMFC are:

-   -   Anode Reaction: CH₃OH+H₂O=CO₂+6H⁺+6e    -   Cathode Reaction: O₂+6H⁺+4e=2H₂O    -   Net Reaction: CH₃OH+3/2O₂=CO₂+H₂O

The methanol in a DMFC is preferably used in an aqueous solution toreduce the effect of “methanol crossover”. Methanol crossover is aphenomenon whereby methanol molecules pass from the anode side of themembrane electrolyte, through the membrane electrolyte, to the cathodeside without generating electricity. Heat is also generated when the“crossed over” methanol is oxidized in the cathode chamber. Methanolcrossover occurs because present membrane electrolytes are permeable (tosome degree) to methanol and water.

One of the problems with using DMFC power systems in portable powerapplications is the lack of a low-cost, effective method and system forremoving effluents produced by the electrochemical reaction generally,and in particular, to remove water generated on the cathodic face of themembrane electrolyte or otherwise present in the cathode chamber. Ifwater generated in the cathode chamber collects on the cathode of themembrane or in the anode chamber, it may prevent oxygen from coming intocontact with the cathodic electrocatalyst, interrupting productiveoxidation of the fuel and generation of electricity.

In addition, the proper ratio of fuel to water delivered to the anodechamber in DMFC power systems must be maintained. During operation,water molecules may be pulled across the membrane with hydrogen protonsleading to excess water on the cathode side of the membrane and anincrease in methanol concentration at the anode. The increasedconcentration of methanol may lead to additional methanol crossoverresulting in decreased efficiency, a waste of methanol, and thegeneration of unwanted heat.

Theoretically, the effluents could be removed by venting the carbondioxide out of the anode chamber and evaporating the water from thecathode side of the membrane electrolyte with a low humidity ambientairflow. However, under many relevant conditions (e.g., low volume airflow, low ambient air pressure, moderate to high humidity), the watercannot be effectively removed, and thus, alternate methods ofeliminating water generated in the cathode are required.

According, the suitability of DMFC power systems for powering portabledevices and consumer electronics is dependent upon the development ofsystems and methods for eliminating and/or recirculating the effluentproducts produced during operation of the fuel cell. In addition, inorder for DMFC power systems to be used effectively, they must beself-regulating and passively generate electrical power under benignoperating conditions, such as ambient air temperature and pressure.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a water management systemand method for managing effluent products generated as a result of fueloxidation in a fuel cell system. More particularly, the presentinvention provides a water management system and method using aneffluent gas (carbon dioxide) generated as a by-product of said fueloxidation to remove or recirculate water from the fuel cell system.

The water management system and method according to the presentinvention is particularly well suited for use with a direct oxidationfuel cell system. Carbon dioxide produced from the oxidation of fuel isnot directly exhausted from the fuel cell system but instead, used toremove/recirculate effluent water.

The present invention also provides a system and method forrecirculating effluent water in a fuel cell system to maintain apreferred concentration of the carbonaceous fuel, thereby reducing theamount of water that must be stored with the carbonaceous fuel tomaintain an optimum fuel concentration.

Accordingly, the below recited aspects of the present invention aredirected to direct oxidation fuel cell systems, and more preferably todirect methanol fuel cell power systems.

In one aspect of the present invention, an effluent gas produced in ananode chamber of a fuel cell is collected and then exhausted through acathode chamber of the fuel cell when the amount of effluent gas reachespredetermined value.

In another aspect of the present invention, a fuel cell includes ananode chamber having a fuel, a cathode chamber in fluid communicationwith an oxidizing agent, a proton conducting membrane electrolyteseparating the chambers, and a first valve for controlling a first flowof a gas from the anode chamber into the cathode chamber. A relatedmethod for reducing the amount of water in the cathode chamber includesclosing the first valve allowing an effluent gas produced in the anodechamber to collect and opening the first valve when an amount of theeffluent gas reaches a predetermined value.

In yet another aspect of the present invention, the fuel cell accordingto the second aspect further includes a gas plenum and a second valve.The first valve controls the first flow of the gas from the anodechamber into the gas plenum and the second valve controls a second flowof the gas collected in the gas plenum into the cathode chamber. Afurther related method includes opening the first valve allowing saidfirst flow while said second valve is closed between said gas plenum andsaid cathode chamber. Effluent gas is then collected in the gas plenumvia the first flow. When an amount of effluent gas collected in the gasplenum reaches a predetermined value, the first valve is closed and thesecond valve is opened, allowing the second flow.

In yet another aspect of the present invention, which may be used inconjunction with the above aspects, a fuel cell includes a fluid plenum,a third valve for controlling the second flow out of an outlet of thecathode chamber and into the fluid plenum and out an exhaust port and afourth valve for controlling a third flow from the fluid plenum into theanode chamber.

The third valve of the fourth aspect allows the second flow between theoutlet of the cathode chamber and the exhaust port when placed in afirst position, and allows the second flow between the outlet and thefluid plenum when placed in a second position.

The fourth valve of the fourth aspect allows the third flow when placedin a first position and allows a fourth flow which controls a flow offuel from a fuel supply cartridge to the anode chamber when placed in asecond position.

In yet another aspect of the present invention, a fuel cell systemincludes an anode chamber having a fuel and a cathode chamber in fluidcommunication with an oxidizer. The cathode chamber includes an inletpositioned in a first end of the cathode chamber and an outletpositioned adjacent a second end of the cathode chamber. The fuel cellaccording to the fifth aspect further includes a proton conductingmembrane electrolyte separating the chambers and having an effluentgas-permeable portion allowing effluent gas produced in said anodechamber to flow into the cathode chamber, and a nozzle having an inletpositioned adjacent the gas-permeable portion in the cathode chamber andan outlet positioned adjacent outlet of the cathode chamber.

In yet another aspect of the present invention, a method for removingwater in a cathode chamber of a fuel cell, the fuel cell including ananode, a cathode chamber having an inlet and an outlet, and a membraneelectrolyte having a gas-permeable portion, the method includesdirecting an effluent gas produced in the anode chamber from thegas-permeable portion into the cathode outlet at a pressure,establishing a low pressure region adjacent the outlet, and inducing aflow from the inlet through the cathode chamber and exiting the outlet.The above aspect may further include equalizing the pressure to anambient pressure adjacent the outlet of the cathode chamber.

The flows recited in the above aspects may be communicated through thevarious elements via conduits and/or channels.

In addition, above aspects may include a controller for actuating thevalves for controlling the flows.

BRIEF DESCRIPTIONS OF THE DRAWINGS

For a better understanding of the invention, reference is made to thedrawings which are incorporated herein by reference and in which:

FIG. 1 is a schematic diagram of a water management system according toa first embodiment of the present invention.

FIGS. 2A-2B are schematic diagrams of modes of gas flow into a directoxidation fuel cell system according to the first embodiment for thepresent invention.

FIG. 3 is a schematic diagram of a water management system according toa second embodiment of the present the invention.

FIG. 4A-4B are schematic diagrams of gas flow and water return into adirect oxidation fuel cell system according to the second embodiment ofthe present invention.

FIG. 4C is a schematic diagram of a controller system for the watermanagement system for the embodiments of the present invention.

FIG. 5 is a schematic diagram of a water management system according toa third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “low humidity gas” as used herein refers to ambient air orother gas containing substantially less than its saturation level ofwater vapor, that is, having a relative humidity of less than 100%.

As shown in FIG. 1, a direct oxidation fuel cell system 20 includes amembrane electrolyte assembly 21 having a proton-conducting,electronically non-conductive membrane electrolyte 26 disposed betweenan anode chamber 22 and a cathode chamber 24. The exact shape of theanode chamber and cathode chamber may be defined by a “flow fieldchannel” which may be integrated into a flow field plate (not shown),which aids in distributing the fuel and the oxidizing agent to themembrane electrolyte. In this diagram, each surface of the membraneelectrolyte 26 is coated with electrocatalysts which serve as anodereactive sites 23 on the anode chamber side of the membrane and cathodereactive sites 25 on the cathode chamber side of the membrane. The anodeand cathode reactive sites facilitate the electrochemical reactions ofthe DMFC.

It is worth noting that the electrocatalysts may be provided in otherareas within the anode and cathode chambers, and thus, the invention isnot limited to fuel cells where the catalysts are provided on themembrane electrolyte.

Diffusion layers 27 and 28, may be included and positioned on eitherside of the membrane. These layers provide a uniform effective supply ofmethanol solution (diffusion layer 27) to the anode reactive sites and auniform effective supply of oxidizing agent (diffusion layer 28) to thecathode reactive sites. Diffusion layers 27 and 28 on each of the anodeand cathode sides of the membrane electrolyte also assist in providingoptimal humidification of the membrane electrolyte by assisting in thedistribution and removal of water to and from the membrane electrolyteat rates that maintain a proper water balance in the DMFC power system.Moreover, each layer may be used with a flow field (not shown), tofurther aid in distributing fuel and oxidizer to the respective reactivesites.

The form of the anode chamber may be defined by a flow field plate (notshown) which guides the fuel mixture over the anode diffusion layer andalso functions as a conductor (i.e., acts as the electrical anode), andan exhaust vent 30 which allows carbon dioxide created during oxidationof the fuel to pass out of the anode chamber. Similarly, the cathodechamber may include a flow field plate (not shown) which guidesoxidizing agent in the chamber and also functions as a conductor (i.e.,acts as the electrical cathode), an inlet 33 and an exhaust outlet 34which allows air to flow through the cathode chamber so that an adequatesupply of oxygen is insured for the reaction. One skilled in the artwill appreciate that air may flow from inlet 33 to outlet 34 and in theopposite direction, when the system is exposed to an ambient airpressure.

In a DMFC power system, an aqueous methanol solution, preferably asolution greater than 0% to about 100% methanol by volume, morepreferably between greater than 0% to about 30% methanol by volume andmost preferably approximately 3% methanol by volume, is used as thecarbonaceous fuel reactant. The methanol solution circulates past theanode reactive sites 23. Upon the application of an electrical loadbetween the flow field plates of the anode and the cathode chambers, themethanol solution disassociates , producing hydrogen protons andelectrons, and generating carbon dioxide as a first by-product of fueloxidation. Hydrogen protons migrate through the membrane electrolyte tothe cathode chamber while electrons pass through the external load. Theprotons and electrons then combine with oxygen in the cathode chamber toform water, the second by-product of the reaction. The electrons areretrieved by the flow field plate of the anode chamber and carriedthrough an external electrical load 29 to the flow field plate of thecathode chamber.

First Embodiment

In a first embodiment of the present invention, the flow of carbondioxide is controlled through selective positioning of a vent valve 32and an air inlet valve 36. The vent valve 32 is a two-way valveincorporated in an exhaust vent conduit 30 for controlling venting andaccumulation of carbon dioxide (in conjunction with the air inlet valve)into a gas plenum 34. The air inlet valve 36 is a three-way valveincorporated at the intersection of an air inlet conduit 33 a and a gasplenum conduit 34 a, for controlling the flow of air and carbon dioxide(in conjunction with the vent valve 32) into the cathode chamber.

The positioning of the vent valve and the air inlet valve determinewhether the water management system operates in an air inlet mode or aflush mode. The air inlet mode allows air from the air inlet conduit 33a to flow into the cathode chamber and out of the exhaust outlet 34carrying water away and refreshing the available oxygen for reaction atcathode reactive sites.

During the flush mode, a significant pressure drop caused by the buildupof carbon dioxide in the fluid plenum produces a high flow velocity ofcarbon dioxide from the gas plenum into the cathode chamber 24. Thispressure drop also reduces the relative humidity of the carbon dioxidestored in the plenum, so that it can more readily absorb water in thecathode chamber. Thus, water is flushed from the cathode chamber bybeing blown out of the chamber by the pressure, and is evaporated due tothe lowered relative humidity of the carbon dioxide.

FIG. 2A illustrates the positions of vent valve 32 and the air inletvalve 36 during an air inlet mode. As shown, the vent valve 32 is openbetween the anode chamber 22 and the gas plenum 34 to allow carbondioxide to accumulate in the gas plenum 34. The air inlet valve 36 isclosed to the gas plenum 34 and open between an air inlet and thecathode chamber 24, so that the plenum can operate as a storage tank forthe carbon dioxide and so that air may flow into the cathode chamber 24as required for fuel oxidation.

FIG. 2B illustrates the positions of the vent valve 32 and the air inletvalve 36 during a flush mode. In the flush mode, the vent valve 32 isclosed between the anode chamber 22 and the gas plenum 34 and the airinlet valve 36 is open to the gas plenum 34 and closed between the airinlet and the cathode chamber. This positioning allows the stored carbondioxide to flow out of the gas plenum and into the cathode chamber viaconduit 40.

The vent valve 32 is preferably actuated to the flush mode positionfirst, or concurrently with the air inlet valve 36. If the air inletvalve 36 is actuated before the vent valve, fuel may be expelled fromthe anode chamber into the cathode chamber adversely affecting theefficiency of the system as fuel is not used to generate power, but iswasted.

Because the membrane electrolyte operates more effectively withincertain humidification parameters, the flush mode will not dehydrate orremove substantially all water from the membrane electrolyte.

In order for the flush mode to operate effectively, a predeterminedsufficient amount of carbon dioxide is necessary to flush the water fromthe cathode reactive sites. Accordingly, the amount of carbon dioxidewhich has been generated must be determined.

In the present invention, the volume of carbon dioxide may be determinedin the following ways:

-   -   (1) the level of fuel solution;    -   (2) the pressure level of the anode chamber;    -   (3) a time interval;    -   (4) power produced; and    -   (5) fuel concentration.

These methods and corresponding systems relate generally to activecontrol of the flow of carbon dioxide to the cathode chamber. Suchactive control is generally managed by a controller (digital or analog)which actuates the valves for the various modes. However, it is worthnoting that flow may also be controlled passively via relief valves,gas-permeable membranes, and other components that are well known tothose skilled in the art.

Fuel Solution Level

As carbon dioxide is created and accumulates in the anode chamber andgas plenum, it pushes against the surface of the fuel solution. Apredetermined displacement of carbonaceous fuel correlates to apredetermined volume of carbon dioxide sufficient to cause or assist inthe removal of water from the cathode chamber. The system, however, isconfigured so that the predetermined displacement which determines whena flush mode is required still allows normal power generation. Thus,electricity production will not diminish as the predetermined level isreached.

Accordingly, when the predetermined displacement level is reached, asensor sends a signal to a controller for actuating the valves to theflush mode positions.

The valves may be reset to air inlet mode by either the sensor, whichindicates that the fuel is no longer displaced the predetermined amount,or by other means, including, but not limited to the use of a timer. Theprocess is then repeated to continue generation of electricity.

Anode Chamber Pressure

A sufficient amount of carbon dioxide may be determined by detecting thelevel of pressure in at least one of the anode chamber 22 and the gasplenum 34. With this method and system, the pressure within the anodechamber is not fixed, rather it increases with the anodic oxidation ofthe fuel solution due to the generation of carbon dioxide within theanode chamber 24.

In the air inlet mode, the pressure of the anode chamber 22 typicallyvaries in relation to the water generated in the cathode chamber 24. Theamount of carbon dioxide in the closed volume of the anode chamber, isdirectly related to the amount of water generated in the cathodechamber. A predetermined level of pressure is associated with an amountof carbon dioxide sufficient to remove said water from the cathodechamber.

When a pressure sensor (e.g., diaphragm type, or resistive bridge)within a wall of the anode chamber detects the predetermined pressurelevel, a controller actuates the valves to the flush mode positions. Itis worth noting that this method and system may not require acontroller. Specifically, the valves may be pressure-responsive releasevalves, actuated in response to a predetermined pressure level, or otherfuel cell system operating characteristics.

Time Periods

Alternatively, the valves may be actuated between an air inlet mode anda flush mode positions after a predetermined period of time has elapsedduring fuel cell operation. The controller tracks the amount of timewhen the cell is used for power. Since the power provided would be at apredetermined voltage/current, the amount of carbon dioxide produced perunit time can be determined. Thus, after a predetermined operation timeperiod has elapsed, the controller will actuate the valves to flush modepositions.

Power Production

In a similar method and system, the carbon dioxide level may bedetermined by tracking how much electric energy has been produced by thecell. Accordingly, a predetermined amount of energy (power produced overa time interval) correlates with a certain amount of carbon dioxidegenerated. The controller tracks the amount of energy output andactuates the valves when the predetermined amount of energy has beenproduced.

Fuel Concentration

The fuel in the anode chamber is a mixture of carbonaceous fuel (i.e.,methanol) and water. Unless otherwise compensated, as the oxidationprocess occurs, the fuel becomes less concentrated in the solution,i.e., less fuel, more water. Because the concentration of thecarbonaceous fuel in the aqueous fuel solution and the amount of carbondioxide generated are inversely related (provided that adjustments forintroducing additional fuel are made), it is possible to determine howmuch carbon dioxide has been generated at the anode by measuring theconcentration of the fuel in aqueous solution. Thus, a fuelconcentration sensor (or sensors) sends fuel concentration signals tothe controller. When the concentration reaches a predetermined minimumindicating that fuel has been consumed to generate a sufficient amountof carbon dioxide to remove water from the cathode chamber, thecontroller actuates the valves to the flush mode positions.

Second Embodiment

FIGS. 3 and 4A-4B illustrate a second embodiment according to thepresent invention. In this embodiment, a recirculation system providesactive control for recirculating at least a portion of the watergenerated or transported across the membrane during fuel cell operation.By returning water to the anode chamber, fuel concentration is kept atan optimum level for efficient fuel cell operation, decreasing thevolume of water that must be stored with the methanol in a fuel supply,thus allowing the DMFC power system to have an increased energy density.The recirculation system is preferably used in conjunction with the gasflow control methods and systems described in the previous embodiment.

The recirculation system includes a drain conduit 50 connected to theoutlet of the cathode chamber, a drain valve 54, a drain outlet 51, afluid plenum 57, a first fluid plenum conduit 52, a second plenumconduit 53, a return valve 56, a fuel supply conduit 59 and an anodesupply conduit 60 connected to an inlet of the anode chamber.

The drain valve 54 is a three-way valve positioned at the intersectionbetween the drain conduit 50, the drain outlet 51 and the first fluidplenum conduit 52, and is used either to exhaust water and gaseouseffluent from the cathode chamber to the environment (connecting drainline 50 to the drain outlet 51), or to recirculate water removed fromthe cathode chamber to the cathode chamber (connecting drain line 50 tothe first fluid plenum conduit 52).

The return valve 56 is also a three way valve positioned at theintersection of the second fluid plenum conduit 53, the anode inletconduit 60 and the fuel supply conduit 59, and is used to keep aspecific amount of fuel solution in the anode chamber (connecting thefuel supply conduit 59 to the anode supply conduit 60), and torecirculate the water recovered from the cathode chamber into the anodechamber (connecting second fluid plenum conduit 53 with the anode supplyconduit 60). The valve 56 may also be used as a mixing chamber, formixing fuel solution with recirculated water from the cathode chamberfor supply to the anode chamber.

Depending upon the state of the system generally, and the distributionof water in the DMFC power system, valves 54 and 56 may be actuatedsequentially or simultaneously. Preferably, valves 54 and 56 are used inconjunction with the previous embodiment, the drain valve 54 and thereturn valve 56 include corresponding positions for the air inlet modeand the flush mode, and thus may be actuated upon detection of the sameprocess variables and/or physical conditions for vent valve 32 and airinlet valve 36. Alternatively, the drain valve 54 and the return valve56 may be activated independently of carbon dioxide flow controlprocess.

FIGS. 4A and 4B illustrate the positions of the drain valve 54 andreturn valve 56 during an air inlet mode (FIG. 4A) and a flush mode(FIG. 4B). As shown in FIG. 4A, during an air inlet mode, the drainvalve 54 exhausts water and carbon dioxide to the environment and thereturn valve 56 is closed to the fluid plenum 57 and open between thefuel supply cartridge 39 and the anode chamber 22. This establishes athroughput between the fuel supply 39 and the anode chamber 22 todischarge pressurized fuel into the anode chamber 22. Alternatively,during the air inlet mode, the return valve 56 (as shown in phantom) mayperiodically close to the fuel supply 39 and open to the fluid plenum 57establishing a throughput between the fluid plenum 57 and the anodechamber 22 to return water to the anode chamber 22 for adjustment of theconcentration of methanol solution. The return valve may also be closedto the fuel supply to halt the admission of new fuel during periods oflow or no power generation.

Upon detection of the sufficient volume of carbon dioxide for the flushmode (FIG. 4B), the drain valve 54 is actuated to open to the fluidplenum 57 establishing a throughput between the cathode chamber 24 andthe fluid plenum 57. Because pressure of the anode chamber 22 is higherthan ambient pressure of the cathode chamber 24 during the air inletmode, upon actuation to the flush mode of gas flow control, the pressureof the anode chamber 22 drops substantially and equilibrates with thepressure of the cathode chamber 24. Water flushed from the cathodechamber 24 in the flush mode, therefore, is propelled by the flow ofcarbon dioxide, assisting in the collection of water in the fluid plenum57.

Generally, since only a portion of water may be required forrecirculation to the anode chamber, the drain valve 54 may remain opento the fluid plenum 57 for comparatively short intervals during theflush mode. Since the cathode and anode chambers, though connectedtogether through valves 32 and 36, are together closed to the ambient byvalves 36 and 54, some water pressure is maintained during recirculationof water from the cathode chamber 54 to the fluid plenum 57.Accordingly, if the drain valve 54 is closed to the plenum 57 (i.e.,allowing fluid communication between drain conduit 50 and the drainoutlet 51), the water within the fluid plenum can be discharged into theanode chamber upon opening of the return valve 56 between the secondfluid plenum conduit and the anode supply conduit. It should beunderstood that additional valve(s) or plenum(s) (not shown) betweenreturn valve 56 and anode chamber 22 may be used to control the flow ofrecirculated water and or fuel into anode chamber.

FIG. 4C illustrates a generic controller for actuating the valves inboth the first and second embodiments (independently and inconjunction). Accordingly, a controller 80, receiving signals fromsensor 90 (i.e., carbon dioxide levels) sends signals to valve actuators82, 84, 86 and 88 at the appropriate times for the air inlet mode, theflush mode, fuel supply and water recirculation.

Third Embodiment

FIG. 5 illustrates a third embodiment according to the presentinvention. In this embodiment, a passive control system using a lowhumidity gas produced in the anode chamber removes water from thereactive sites in the cathode chamber using an enhanced air flow system.

The system according to this embodiment includes a membrane electrolyteassembly 62 disposed between an anode chamber 70 and a cathode chamber72 of a fuel cell system 60. The assembly 62 includes aproton-conducting, electronically non-conductive membrane electrolyte 64having a gas-permeable sector 66 selectively permeable to a desiredeffluent gas, such as carbon dioxide, but not to water or fuel. A gasejector 68 is connected to the cathode side of the gas-permeable sector66.

The gas ejector 68 having a first end 67 and a second end 69, may beconstructed preferably in a conical, parabolic or exponential shape.Each of these shapes causes the acceleration of the flow velocity ofcarbon dioxide as it travels from the first end to the second end. Toattain such a flow profile, the broadest portion of each shape ispositioned on the first end, with the narrowest portion positioned atthe second end.

The gas ejector 68 further includes a collar 65 disposed at the secondend 69 of the gas ejector 68, and is preferably positioned to encompassa low pressure region at the exit of the gas ejector, created by theflow of gas through the ejector. As air flows toward the low pressureregion, the air flow is entrained in the low pressure region. The collar65 then carries the entrained air, together with any water flushed fromthe cathode chamber and other effluents, toward an outlet 79.

Accordingly, the system operates in the following manner. Air, suppliedto the cathode chamber 72 from an external source, is delivered by anair inlet 74 to the cathode chamber. The low pressure region located atthe second end 69 of the gas ejector draws air from the air inlet 74toward the low pressure region. Air thereby is forced to flow into andthrough the cathode chamber 72 at an enhanced velocity such that excesswater accumulating in the cathode chamber, especially at the cathodereactive sites, is flushed or evaporated out of the chamber.

Having thus presented the present invention in view of the abovedescribed embodiments, various alterations, modifications andimprovements will readily occur to those skilled in the art. Suchalterations, modifications and improvements are intended to be withinthe scope and spirit of the invention. Accordingly, the foregoingdescription is by way of example only and is not intended as limiting.The invention's limit is defined only in the following claims and theequivalents thereto.

1-55. (canceled)
 56. A direct oxidation fuel cell system, comprising: acatalyzed membrane electrolyte separating an anode chamber and a cathodechamber, wherein the anode chamber has no liquid exit port; a fuelsource for providing fuel to the anode chamber, wherein the fuelcomprises essentially methanol; and a load coupled across the fuel cell,providing a path for electrons produced in electricity-generatedreactions of the fuel cell.
 57. The system of claim 1, wherein theessentially methanol comprises greater then 90% methanol by volume. 58.The system of claim 1, wherein the essentially methanol comprisesgreater then 95% methanol by volume.
 59. The system of claim 1, whereinthe essentially methanol comprises greater then 98% methanol by volume.60. The system of claim 1, wherein the essentially methanol comprisesapproximately 99% methanol by volume.
 61. The system of claim 1, whereinat least a portion of one wall of the anode aspect is gas permeable andliquid impermeable
 62. A method of delivering fuel to a direct oxidationfuel cell, comprising: providing a direct oxidation fuel cell includinga catalyzed membrane electrolyte, having an anode aspect and a cathodeaspect; providing a fuel to the anode aspect of the catalyzed membraneelectrolyte, the fuel comprising methanol to about 99% by volume; andcollecting an effluent gas produced in an anode chamber of the fuel celland exhausting the collected gas through a cathode chamber to an ambientenvironment.
 63. A method of delivering fuel to a direct oxidation fuelcell, comprising: providing a direct oxidation fuel cell including avalve for controlling a flow of a gas from an anode chamber to a cathodechamber and a catalyzed membrane electrolyte, having an anode aspect anda cathode aspect; and providing a fuel to the anode aspect of thecatalyzed membrane electrolyte, the fuel comprising methanol to about99% by volume.
 64. A method for delivering fuel to a direct oxidationfuel cell system, comprising: providing a catalyzed membrane electrolyteseparating an anode chamber and a cathode chamber, wherein the anodechamber has no liquid exit port; providing a fuel source for providingfuel to the anode chamber, wherein the fuel comprises essentiallymethanol; and providing a load coupled across the fuel cell, providing apath for electrons produced in electricity-generated reactions of thefuel cell.
 65. The method of claim 9, further comprising: deliveringfuel by a direct fuel feed into the anode chamber without anoderecirculation.
 66. The method of claim 9, further comprising: providingat least a portion of one wall of the anode chamber is gas permeable andliquid impermeable.
 67. The method of claim 9, wherein the essentiallymethanol comprises greater then 90% methanol by volume.
 68. The methodof claim 9, wherein the essentially methanol comprises greater then 95%methanol by volume.
 69. The method of claim 9, wherein the essentiallymethanol comprises greater then 98% methanol by volume.
 70. The methodof claim 11, wherein the essentially methanol comprises approximately99% methanol by volume.
 71. A method of delivering fuel to a directoxidation fuel cell, comprising: providing a direct oxidation fuel cellincluding a valve for controlling a flow of a gas from an anode chamberto a cathode chamber and a catalyzed membrane electrolyte, having ananode aspect and a cathode aspect; and providing a fuel to the anodeaspect of the catalyzed membrane electrolyte, the fuel comprisingmethanol to greater then 95% by volume.